Problem: On the first 4 quizzes of her math class, Nadia got an average score of 88. What does she need on the next quiz to have an overall average of 90?
Explanation: Let her score on the next quiz be $x$ The sum of all of her scores is then $4 \cdot 88 + x$ The same sum must also be equal to $5 \cdot 90$ Solve: $x = 5 \cdot 90 - 4 \cdot 88 = 98$.